I always prefer to not count parts, period. Generally, I don't have too many problems with customers with this method....I explain to them that the cost of my counting the parts (to them) is much more expensive than the cost of them supplying excess parts on the reel at a cost of $9/reel. Obviously, more expensive parts are treated differently.
If you have to count parts for inventory cycle counts, or a customer that's being very particular about it, a reel counter as shown in a previous post is your best bet. If that's not financially viable, you can use the percentages listed on the reels as a pretty good approximation of the quantities remaining. If that's not accurate enough, you can go to the measure/calculate method. If you search the SMTNet archives you'll find a thread on the formula for calculating the amount of parts left on a reel based on the length of the tape. Finally, one other method that has worked well and quickly for me in the past is to count out say 25 parts, and mark that length of tape on a piece of scrap paper taped to a bench. Run the tape over it and count the number of lengths you have.